As a European private network communication standard recently issued by ETSI (European Telecommunications Standards Institute) for taking place of the analog Private Mobile Radio (PMR), the Digital Mobile Radio (DMR) standard is advantageous in large coverage area, high transmission rate, high spectrum efficiency and excellent energy-saving efficiency. Therefore, private network communication products based on DMR standard have a promising future in the market.
The DMR standard uses Time Division Multiple Access (TDMA) frame structure with double time slot. FIG. 1 is a schematic view of DMR TDMA frame structure in direct mode. The so-called direct mode means that communication terminals communicate with each other directly without a relay station. As shown in FIG. 1, the DMR TDMA frame includes two completely identical time slots with each in a length of 30 ms, and each of the time slots is further divided by a middle synchronous pattern field into two loads with the same length. Each of the time slots is provided with a protective interval in a length of 1.25 ms at two ends thereof. Accordingly, the interval between the time slots is 2.5 ms.
In relay mode, Common Assignment Channel (CACH) information is inserted into the DMR TDMA frame illustrated in FIG. 1 by the relay station to broadcast time slot number and occupant condition of each time slot. FIG. 2 is a schematic view of DMR TDMA frame structure at a down direction in relay mode. As shown in FIG. 2, CACH information in a length of 2.5 ms is inserted by the relay station into a protective internal in a length of 2.5 ms between the two time slots, and the information is used to indicate the time slot number and the occupant condition of the subsequent time slot. Accordingly, there is CACH information instead of a protective interval between time slots in relay mode.
In relay mode, the existing implementation method for the DMR private network communication sets a communication time slot for a communication terminal in advance, and once set, the communication time slot for one communication terminal cannot be changed. For the purpose of ensuring that each of the communication terminals in the same group can communicate with each other, the communication time slot assigned in advance for each of the communication terminals in the same group is the same. Meanwhile, in order to ensure no interference of communications among different groups, communication terminals with different communication time slot are used by different groups. In a process of communication, the relay station periodically transmits CACH information for indication of time slot number and occupant condition of each time slot. The communication terminal receives the CACH information to establish synchronization with the information, thereby locking the preset communication time slot to perform private network communication by the time slot. In this case, different groups perform communication by different time slots, thereby making full use for two time slots in the DMR TDMA frame.
The above implementation method is disadvantageous in that the communication terminal can only perform communication by the preset communication time slot thereof. Accordingly, communication terminals with different communication time slots cannot communicate with each other. As a result, communication terminals in the same group must be provided with the same time slot for communicating with each other. In this case, although two independent channels are set by the DMR protocol itself in a frequency width of 12.5 KHz to support communications of two independent private networks, only one fixed time slot can be used for communications in a group, while the other time slot cannot be used even if it is idle. Accordingly, only a frequency width of 6.25 KHz can be used in one group, and the other frequency width of 6.25 KHz in the 12.5 Khz cannot be used.
Hence, there is need for an implementation solution for the DMR private network communication which is capable of overcoming the defects existed in the prior art.